Due to the spread of DVDs (digital versatile disks), large-screen televisions, projectors, and the like, home theaters can now be readily enjoyed. In order to create a larger screen, images may be projected from a front projector onto a screen which is two to three meters from the projector, thereby producing a large 80- to 100-inch image.
In theaters, sound is as important as images, and at home there is a need to produce a sound source at the screen or in the vicinity of the screen as in the theater, so as to improve presence. Superdirectional acoustic systems have been developed, such as acoustic systems using ultrasonic speakers for producing a virtual sound source on a projector screen (see Japanese Unexamined Patent Application, First Publication No. S60-254992), and projectors which include ultrasonic speakers (see Japanese Unexamined Patent Application, First Publication No. H11-262084).
FIG. 7 shows the structure of a conventional ultrasonic speaker, which includes an audio frequency wave (signal) oscillating source 81 for generating a signal in an audio (or human-audible) frequency band, a carrier wave (signal) oscillating source 82 for generating a carrier wave (signal), a modulator 83, a power amplifier 84, and an ultrasonic transducer 85.
In the above structure, by using a signal output from the audio frequency wave oscillating source 81, the carrier wave in an ultrasonic frequency band, output from the carrier wave oscillating source 82, is modulated in the modulator 83, and the ultrasonic transducer 85 is driven by the modulated signal which has been amplified by the power amplifier 84. Accordingly, the modulated signal is converted by the ultrasonic transducer 85 into a sound wave having a finite amplitude level. The sound wave is emitted into a medium (i.e., the air), thereby reproducing the original signal sound at the original audio frequency due to the non-linear effect of the medium (i.e., the air).
In this case, the reproduction area of the reproduced signal in the audio frequency band is in the form of a beam extending from the ultrasonic transducer 85 along the emission axis.
FIGS. 8A and 8B show the structures of ultrasonic transducers used in conventional ultrasonic speakers. Most conventional ultrasonic transducers are resonant transducers using piezoelectric ceramics (i.e., an piezoceramic element) as an oscillation element. The ultrasonic transducers shown in FIGS. 8A and 8B perform both conversion of electrical signals into ultrasonic waves and conversion of ultrasonic waves into electrical signals (i.e., sending and receiving of the ultrasonic wave), by using a piezoceramic element as the oscillation element.
The bimorph ultrasonic transducer shown in FIG. 8A has two piezoceramic elements 91 and 92, a cone 93, a case 94, leads 95 and 96, and a screen 97. The piezoceramic elements 91 and 92 are adhered to each other, and the leads 95 and 96 are respectively connected to the faces of the piezoceramic elements 91 and 92, on the opposite sides of the adhesion faces.
The unimorph ultrasonic transducer shown in FIG. 8B has a piezoceramic element 101, a case 102, leads 103 and 104, inner wiring 105, and a glass member 106. The lead 103 is connected via the inner wiring 105 to the piezoceramic element 101 which is grounded via the case 102.
The resonant transducer uses a resonance phenomenon of piezoelectric ceramics; thus, preferable ultrasonic transmitting (and receiving) characteristics are obtained only in a relatively narrow frequency range in the vicinity of the resonance frequency, so that sound or tone quality is inferior.
On the other hand, when an audio signal is reproduced using a projector which has the above-explained ultrasonic speaker, the listener hears the reproduced sound which was reflected by the screen. However, the sound range which is reproducible using an ultrasonic speaker as a superdirectional speaker is limited to a relatively high frequency range. Therefore, the reproduced sound including relatively weak low frequency sound has inferior sound presence.